It is well known that instantaneous or limiting values of a measured quantity may be indicated acoustically. In particular, an acoustic indicator has been proposed (DE-PS 2363731) for portable measuring and warning apparatus for measuring exposure to radiation. Such units may be clipped on to clothing and furnish a rough indication of the instantaneous value of the measured quantity. They can also generate a warning tone when the measured quantity exceeds a limiting value. The measured value may, for example, be indicated by different frequencies or by a different sequence in time of acoustic pulses. Acoustic limit indicators have been proposed for electrical measuring instruments (U.S. Pat. No. 4,321,530), torque meters (U.S. Pat. No. 4,244,434) and meters for controlling the axle load of trucks (DE-OS 2729380). In all of these, an acoustic signal is generated when the quantity being measured exceeds the predetermined limiting value.